In recent years, wireless LAN apparatuses using the IEEE802.11 standard have been widely used as radio network devices. The application range of the wireless LAN apparatuses is expanding from conventional use for personal computers to use for portable devices including mobile phones. There is a high demand for power-saving and highly efficient communication in these portable devices in particular. In the present specification, the IEEE802.11 standard will be referred to as “802.11 standard.”
The 2.4 GHz band among frequency bands used by wireless LAN apparatuses compliant with the 802.11 standard can be operated without licenses, for example, in Japan, and many wireless network apparatuses are being standardized. For example, Bluetooth (Bluetooth is a registered trademark), which employs a frequency hopping scheme (FHSS: Frequency Hopping Spread Spectrum), is widespread as wireless apparatuses using the 2.4 GHz band. Moreover, at ordinary homes, microwave ovens emit electromagnetic wave noise in the 2.4 GHz band and cordless telephone sets of other frequency hopping schemes also use the 2.4 GHz band.
FIG. 15A illustrates the 2.4 GHz band used in the 802.11 standard. A wireless LAN (wireless apparatus) of the 802.11 standard selects a use channel from among 13 channels set at intervals of 5 MHz, and performs carrier sensing multiple access (CSMA) in the physical layer. In CSMA, the wireless apparatus carrier-senses the band before transmission, performs transmission if no signal from other devices is detected and refrains from transmission if a signal higher than a certain threshold level is detected. In the 802.11 standard in particular, it is necessary to abide by a rule called “clear channel assessment (CCA)” associated with preamble detection. In CCA, when carrier sensing is performed on a certain channel and if a preamble of a signal emitted from another apparatus of the 802.11 standard is detected, such a situation is considered CCA busy regardless of the received electric field strength, and the wireless apparatus must refrain from using the channel. That is, while another nearby 802.11 standard apparatus is performing transmission, the wireless apparatus cannot perform transmission even when the received electric field strength is small.
Moreover, the 802.11 standard defines that an interval equivalent to two or more channels should be kept between apparatuses in order to prevent inter-channel interference with other 802.11 standard devices. When, for example, a signal that seems to be transmitted from another 802.11 standard device is detected on [Ch1], adjacent [Ch2] and [Ch3] cannot be used. On the other hand, when a signal that seems to be transmitted from another 802.11 standard device is detected on [Ch5], adjacent [Ch4], [Ch6] and [Ch7] cannot be used.
FIG. 15B illustrates a situation in which a Bluetooth (BT) signal is causing interference among 802.11 channels. In Bluetooth, carrier sensing is also performed before determining a channel to be used, and a channel to be used is determined in FHSS. However, overlapping with a frequency used by the 802.11 standard occurs at a certain probability and interference caused by the overlapping cannot be avoided. The threshold used in carrier sensing of Bluetooth is not as stringent as that of the 802.11 standard. For this reason, even when a signal of another apparatus having a higher level than the level at which the 802.11 standard withholds transmission is detected, transmission may be performed if the level is equal to or lower than a threshold defined in Bluetooth. Thus, although the use of 2.4 GHz band requires no licenses, if another wireless apparatus such as a Bluetooth device, or a microwave oven is located in the neighborhood, radio wave interference may reduce performance in terms of communication speed, and if a Bluetooth device starts transmission or a microwave oven starts operation in the neighborhood, there may a situation where transmission cannot be started until the interference source no longer exists.
When a microwave oven is used in the neighborhood, a strong interfering wave of the 2.4 GHz band is emitted intermittently. During a period of emission of the interfering wave, an 802.11 standard apparatus cannot start radio communication within a range affected by the interfering wave. For this reason, in order to achieve power-saving and highly efficient radio communication using the 2.4 GHz band, it is important for the wireless communication apparatus to detect an interfering wave emitted from a microwave oven and select a use channel while evaluating the magnitude of influence on communication of the wireless communication apparatus.
Conventionally, the following wireless communication apparatuses are used to predict this kind of a microwave oven interfering wave (e.g., PTL 1). In the case of an interfering wave emitted from an ordinary microwave oven, a period during which an interfering wave is emitted with a high frequency and a period during which no interfering wave is emitted are periodically repeated in synchronization with a frequency of a commercial AC power supply or a frequency of an integer multiple thereof during operation of the microwave oven. In the wireless communication apparatus shown in PTL 1, if a received radio wave is from a microwave oven, an RSSI signal indicating the intensity of the received radio wave received via an antenna indicates periodicity made up of a period during which interfering wave is emitted with a high frequency and a period during which no interfering wave is emitted. According to PTL 1, the wireless communication apparatus can detect, through detection of the periodicity of the RSSI signal, timing at which an interfering wave is emitted and timing at which an interfering wave stops, and can thereby predict a time zone in which the microwave oven emits an interfering wave.
In addition, PTL 2 discloses a wireless communication apparatus that detects, when selecting a communication channel to be used by a base station apparatus (base station) from among a plurality of channels, a channel being used by another base station apparatus first, and then detects the received signal intensity of a radio wave transmitted from the other base station apparatus on the detected channel in use, and selects, when there is no communication channel whose received signal intensity falls to or below a predetermined value over a predetermined bandwidth, a communication channel to be used by the base station apparatus in accordance with the received signal intensity of the channel in use.
Moreover, PTL 3 discloses a wireless communication apparatus that measures, when a base station appropriately selects a combination of channels to be used, the intensity of a received signal of a radio wave detected in an operating frequency band, extracts the measured data corresponding to a base station in another radio communication system from the measured data of the received signal intensity, calculates an index value indicating received signal intensity of an interference signal of each channel based on the received signal intensity included in the extracted measure data, performs weighting addition on index values of the respective channels based on a predetermined inter-channel interference table, and selects a channel combination having a minimum influence of interference from the predetermined channel combination configured of frequency channels without frequency overlapping based on the level of interference among the respective frequency channels.